Method for producing a high strength bolt

ABSTRACT

A method for producing a high strength bolt from a carbon steel or a low alloy steel material, which comprises the steps of subjecting said material, in turn, to cold working at the reduction-of-area percentage of 10 percent and over rapid heating to a temperature range from 450* C to A1 transformation point, warm-forming to a bolt shape and air-cooling or cooling at a cooling rate higher than that of the air-cooling. The steel material adapted for use herein includes a steel having a pearlite structure or a tempered martensite structure, and particularly the steel having the latter structure presents excellent resistance to the delayed rupture phenomenon with an accompanied high tensile strength of over 100 kg/mm2, particularly, in the range from 130 to 140 kg/mm2 and over.

United States Patent [191 Shimizu et al.

[451 Apr. 15, 1975 [75] Inventors: Kiyoshi Shimizu; Toshihiro Minami,

both of Kobe; Eiji Niina, Nishinomiya, all of Japan [73] Assignee: KobeSteel, Ltd., Kobe, Japan [22] Filed: Sept. 4, 1973 [21] Appl. No.:394,134

[30] Foreign Application Priority Data 3,674,570 7/1972 Hallstrom et al.148/12 Primary ExaminerLowell A. Larson Attorney, Agent, or Firm-Oblon,Fisher, Spivak, McClelland & Maier [57] ABSTRACT A method for producinga high strength bolt from a carbon steel or a low alloy steel material,which comprises the steps of subjecting said material, in turn, to coldworking at the reduction-of-area percentage of 10 percent and over rapidheating to a temperature range from 450C to A transformation point,warmforming to a bolt shape and air-cooling or cooling at a cooling ratehigher than that of the air-cooling. The steel material adapted for useherein includes a steel having a pearlite structure or a temperedmartensite structure, and particularly the steel having the latterstructure presents excellent resistance to the delayed rupturephenomenon with an accompanied high tensile strength of over 100 kg/mmparticularly, in the range from 130 to 140 kg/mm and over.

10 Claims, 5 Drawing Figures C(AISI I027 STEEL) 0F BOLTS SUBJECTED T0WARM FORMING AT 550C PATENTEDAPR I 51975 3,877 281 SHEET 1 0T 3 FIG. 1A

MATERIAL SUBJECTED T0 43% WIRE- DRAWING (YP76.5 TS88.4 EL|7.4 RAGLO)FIG. 1B

MATERIAL SUBJECTED T0 43% WIRE- DRAW- ms AND THEN STRENGTHENINGTREATMENT AT 550% (YPns T5925 EL2|5 mgmismzsms mfg? 3 3.877. 281

HEAD PORTION SHANK PORTION TEMPERATURE m WARMFORMING (c) FIG. 2

METHOD FOR PRODUCING A HIGH STRENGTH BOLT BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to a method for producinga high strength bolt from a carbon steel or a low alloy steel materialwhich has been subjected to hot-rolling, normalizing or annealing,thereby presenting a ferritepearlite structure or which has beensubjected to hardening and tempering, thereby presenting a temperedmartensite structure, and more particularly to a method which comprisesthe steps of subjecting said steel material, in turn, to cold working ata reductionof-area percentage of percent and over, rapid heating to atemperature range from 450 C to A transformation point, hot forming to abolt shape and air cooling or cooling at a cooling rate higher than thatof the air-cooling, thereby presenting a high strength bolt having atensile strength of 70 kg/mm and over. (Meant by the term, strengtheningtreatment as used herein is a combination of working and heat-treatmentwhich includes a cycle of cold working, rapid heating and rapidcooling.)

2. Description of the Prior Art I-Iithereto, a method for producing ahigh strength bolt has recourse to the steps wherein, to form a boltshape, a cold-or-hot-working or machining operation is used, followed bya refining treatment such as hardening and tempering, thus obtaining adesired strength and toughness.

However, such a method for producing a high strength bolt tends to incurproblems in quality assurance of products. The conventional methodnecessarily dictates the use of a refining treatment at an elevatedtemperature, such as hardening and tempering after forming into a boltshape, such that such a refining treatment results in the necessity forclose adjustment of the atmosphere used in a furnace. Unless such anatmosphere furnace is used, there will result an oxidation anddecarbonization phenomena on the surface layer of the bolt, therebybringing about a wider range of variation in strength with the resultantfailure to present a stable level of quality for the bolts produced.

In general, the production of a high strength bolt having a tensilestrength exceeding the range of 120 kg/mm to 130 kg/mm has been deemeddifficult, because when such a bolt is subjected to tension under asubstantial static load for a certain period of time, there tends tooccur a delayed rupture phenomenon due to a sudden occurrence ofembrittlement, with the appearance thereof exhibiting no plasticdeformation.

Accordingly, it remains desirable to have a method for producing a highstrength bolt which has a high tensile strength but does not create adelayed rupture phenomenon due to the sudden occurrence ofembrittlement.

SUMMARY OF THE INVENTION It is accordingly an object of the presentinvention to provide a method for producing a high strength bolt whichdispenses with the use of a refining treatment using an atmospherefurnace as well as heat-treatment at an elevated temperature, yetproviding the quality equivalent or superior to that of a high strengthbolt which is produced using a conventional refining treatment.

Briefly, according to one embodiment of the present invention, there isprovided a method for producing a high strength bolt from a carbon steelor low alloy steel material, wherein, for the production of a highstrength bolt having a tensile strength ranging from to kglmm a steelwire (rod) as rolled or as normalized and annealed is in turn subjectedto cold working at a reduction-of-area percentage of 10 percent andover, heating at a heating rate of over 50C/min. to a temperature rangefrom 450 C to A, transformation point by using heating means such asresistance heating under air, high frequency induction heating, flameheating, or the like, and then hot-forming to a bolt shape, followed byair cooling or cooling at a cooling rate higher than that of the aircooling. For the production of a high strength bolt having a tensilestrength of over 100 kg/mm a carbon steel or low alloy steel material ina wire or rod form is subjected in turn to a refining treatment ofhardening and tempering, cold working at a reduction-of-area percentageof 10 percent and over, heating at a heating rate of over 50 C/min. to atemperature range from 450 C to A transformation point by using theaforesaid heating means, Warm-forming to a bolt shape and finallyair-cooling or cooling at a cooling rate higher than that of the aircooling. The level of tensile strength referred to herein is notnecessarily limited to that exceeding 110 kg/mm A high strength bolt,which is made of carbon steel or low alloy steel material and which isproduced according to the method of the present invention and has atensile strength of over 110 kg/mm presents excellent resistance to thedelayed rupture phenomenon due to a sudden occurrence of embrittlement,and particularly presents a bolt which has a tensile strength of a rangefrom to kg/mm and over, affording excellent resistance to the delayedrupture pheonomenon, as compared with conventional bolts subjected tothe refining treatment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electron-microscopicpicture at 22,50OX, of the structure of a steel A as rolled and thestructure of the steel A which has been subjected to 43 percentwire-drawing, followed by a strengthening treatment at 550C;

FIG. 2 is a plot showing the relationship of the temperature used inhot-forming a rolled steel A to a bolt shape versus the hardness of abolt at bolt head and shank;

FIG. 3 is a diagram showing distribution of hardness at bolt head andshank, in connection to the bolt produced according to the presentinvention and the bolt produced according to the prior art(cold-forming); and

FIG. 4 is a diagram showing distribution of hardness at bolt head andshank of a bolt produced from the refined steel C according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The types of steelsapplicable to the method of the present invention should preferably becarbon steels having a carbon content of no more than 0.5 percent andlow alloy steels.

The reason why the content of carbon should be no more than 0.5 percentis as follows: Although a high strength bolt having a tensile strengthof 100 to 120 kg/mm may be produced of rolled or tempered steels, theuse of low and medium carbon steels rather than high carbon steels isbest suited and advantageous for the method of the present inventionusing the strengthening treatment from the viewpoints of tensilestrength and toughness. Carbon is an essential component for a materialfor use in producing a high strength bolt having a tensile strength ofover 110 kg/mm for the purpose of applying a refining treatment ofhardening and tempering as well as for achieving arequired hardenabilityand strength. However, from the viewpoint of the delayed rupturephenomenon, the content of the carbon should preferably be no more than0.5 percent. Although the content of other elements to be added are notspecifically limited, it is preferable that a small amount of elementssuch as Al, N, Ti, Nb, etc. may be added for achieving a finer grainsize of austenite crystals.

The steel material, of which the high strength bolt having a tensilestrength of over 110 kg/mm is made, has a tempered martensite structurewhich has been subjected to hardening and annealing treatments. In thisrespect, it should be noted that the tempering to be used depends ondesired strength and toughness of the steel, and the workability of thesubsequent working steps of the steel material, and it should also berecognized that the hardening treatment is effective for achieving thedesired resistance to the delayed rupture pheonomenon, if the grain sizeof the austenite crystals over ASTM No. is obtained by utilizing a rapidheating for achieving finer grain size, in addition to the ordinaryheat-treatment.

The reason why the reduction-of-area percentage of the cold workingshould be over 10 percent is that an increase in strength of a boltshank portion, which has been warm-worked, may be expected from a rapidheating by using air-atmosphere resistance-heating and high frequencyinduction heating during the warmworking, whereas the strength in thebolt head portion will be decreased due to the aforesaid warm-working.As a result, in order to accommodate such incompatibility, thereduction-of-area percentage of 10 percent and over is required forachieving uniformity in the percent is difficult to apply to amartensite steel. For this reason, the reduction-of-area percentage ofcold working of over 10 percent is suited for rolled steel material ornormalized steel material, while the reductionof-area percentage of coldworking of 10 percent to percent is suited for a hardened and temperedsteel. Cold working as used herein should preferably be cold drawing(wire drawing), but is not limited thereto. For instance, the coldworkingused may include roller-dies working.

The temperature as used for heating in the present invention should fallin a range from 450 C to A transformation point. This is because theheating temperature of a range from 449 C to 250 C tends to causecracking in the head portion of a bolt, since the heating temperature iswithin the range of blue shortness temperatures, while heatingtemperatures of over the A, transformation point will lead to a poorstrengthening effect. Meant by the heating temperature is thetemperature obtained by using electric-resistance heating means, highfrequency induction heating means, etc.

Alternatively, in case the strength of a bolt thus produced can notsatisfy the value required, then the bolt may be rapidly heated in asalt bath, lead bath, air atmosphere furnace, or the like, which hasbeen maintained at a temperature ranging from 450C to 700C, and soakedfor no more than 5 minutes therein, followed by air cooling or coolingat a cooling rate higher than that of the air cooling, thereby achievingthe desired strengthening effect.

As is apparent from the foregoing description, the method of the presentinvention can obviate the use of heat treatment at an elevatedtemperature, such as hardening and tempering, after forming to a boltshape as in the conventional method. In addition, since the temperaturefor warm-forming or re-heating treatment is limited to 700C, which ismuch lower than the hardening temperatures, there can be achievedimprovements in the skin condition as well as dimensional accuracy forthe bolts, without incurring the possibility of oxidation ordecarbonization, thereby enabling the production of consistent highquality bolts.

The present invention will now be described in more detail withreference to the ensuing examples.

Table 1 shows the chemical composition of steel wires tested.

Table 1 Chemical Composition of Samples Si Mn P S C Cr Mo B A A151 1024Steel 0.22 0.28 1.55 0.021 0.020 A151 1030 Steel 0.38 0.23 0.75 0.0220.016 A151 1037 Steel 0.25 0.23 1.51 0.017 0.015

AlSl 1035 Steel 0.35 0.24 0.84 0.016 0.024 0.13 0.23 0.0016 A181 4510Steel 0.37 0.26 0.76 0.014 0.020 1.07 0.20

strength of a bolt. Furthermore, although the efficacy of thestrengthening treatment, according to the method of the presentinvention, depends on the types of the materials to be worked, theincrease in the strength of a bolt is lower in the case of thereductionof-area percentage of below 10 percent, while the working atthe reduction-of-area percentage of over 40 In the above Table l, steelsA and B represent the samples which were subjected in turn to coldwiredrawing at a reduction-of-area percentage of 5 to 43 percent,heating at a heating rate of 40C/sec. by using an electric heatingmeans, warm-forming to M10 bolt (shank: 48 L) and water-quenchingimmediately thereafter. In addition to thisQthe bolts thus produced wereagain placed in a salt bath which had been heated to a temperature of550C and held therein for 20 seconds, followed by water quenching.

The shank portions of the M bolts were machined to a diameter of 8.0 andthen subjected to No. 4 tensile test applicable to bolts. The results ofthe test are shown in Table 2.

Table 2 i Reduction-of-area percentage of cold wire-drawing versusmechanical properties Reduction Yield Tensile Elonga- Final TypeProduction -of-area point strength tion reduction of 7: of cold 4\/-of-area Steel condition wire dra- (kg/mm (kglmm (71) percenwing (71)tage Hot-forming 10 69.6 77.2 27.0 62.3

AlSl 76.5 87.0 24.0 61.5

Hot-forming 66.7 75.6 28.0 64.2

and reheat- 74.8 81.0 25.5 62.0

ing treat 20 77.8 87.5 25.0 59.5

ment 77.8 86.5 23.8 58.7

Hot-forming 10 69.6 81.8 18.0 42.7

Hot-forming 5 67.1 77.8 20.0 45.0

and reheat- 10 70.6 82.0 18.0 43.7

ing treat- 72.4 83.6 18.5 41.5

ment 20 77.2 90.9 15.0 38.6

As can be seen from Table 2, cold wire-drawing prior to warm-formingexerts a great effect on the mechanical properties of bolts which havebeen warm-formed. The tensile strength and yield point show an increasewith an increase in the reduction-of-area percentage of cold working,despite the heating used for warmforming. On the other hand, elongation,and toughness required for the final reduction-of-area percentage show atendency to slightly decreasewith the increase in strength. Another factis that bolts, which have been re-heated, present improved strength, ascompared with those which have not been subjected to re-heatingtreatment.

FIG. 1 shows the relationship of the mechanical properties and theelectron-microscopic pictures of materials which are made of rolledsteel of sample A and have been subjected to 43 percent cold wireparedwith a cold worked steel. Thus, the aforesaid fact is considered to havebearing on the increase in strength of bolts, because the shank portionof a bolt does not undergo the influence by hot-forming and henceretains improved properties given by the strengthening treatment of thepresent invention.

Table 3 Heating Rate Versus Mechanical Properties Tpye of Heating YieldTensile Elonga- Final Reduc- Steel Rate Point Strength tion tion-of-area(kglmm (kg/mm 4 A percentage(%) C/sec. 83.7 97.3 21.0 42.0 83.3 96.521.3 42.3 40C/sec. 83.0 97.0 21.0 41.8 82.8 96.3 21.5 42.5

Table 3 Continued Heating Rate Versus Mechanical Properties Yield FIG. 2shows the relationship of a warm-forming temperature versus hardness ofhead portions and shank portions of bolts which were produced bysubjecting steel A, in turn, to cold wire-drawing at a reduction-ofareaof 20 percent, rapid heating at a heating rate of 40C/sec., and formingto M10 bolt, followed by water quenching.

FIG. 2 reveals that the relationship of the difference inreduction-of-area percentage is maintained in fact for the difference inhardness for the bolts which have been cold formed, because the headportion of a cold formed bolt has been subjected to cold working atabout 75 percent reduction-of-area percentage, whereas the shank portionof the bolt has been subjected to cold wire drawing of only percentreduction-of-area percentage. In such cold working, the head leastdifference in hardness between the shank and head portions of the bolt.With such a bolt, there occurs normal rupture at the thread portion,when the bolt ruptures due to a great load. Accordingly, such a bolt ispreferable. The cooling after warm-forming should terminate as rapidlyas possible. In other words, if a bolt is slowly cooled, then the boltwill be annealed, thereby failing to present the tensile strength ofover 80 kg/mm which is essential for the high strength bolt.

Table 4 shows the tensile test results of bolts which have been producedby heating the steel B to 550C at a heating rate of C/sec. andhot-forming the same to M10 bolt, and subjecting the bolt to three typesof cooling, i.e., water-quenching, air-cooling and slow cooling. Thistest reveals that the slow cooling results in the failure to obtain ahigh strength bolt.

Table 4 Cooling Conditions After Hot-Forming Versus MechanicalProperties portion of the bolt gives extremely great hardness and highstrength as compared with those of the shank portion, while presentingreduction in toughness therewith. Accordingly, it is not preferable thatthe rupture occurs at the neck portion of the bolt, in case the bolt .issubjected to a great load and broken, in contrast to the normal ruptureat the thread portion. As the forming temperature is increased, thereresults a lesser difference in hardness between the head and shank por-"tions of the bolt. Although the increase in hardness of the shankportion of the bolt depends on the reductionof-area percentage of coldworking, soaking time at a heating temperature and cooling rate, such anincrease shows a peak at 400C and thereafter gradually goes down as thetemperature becomes higher than the peak temperature. On the other hand,the hardness of the head portion of the bolt varies, to some extent,with the varying reduction-of-area percentage of cold-wiredrawing andcompression percentage of the head portion of the bolt, while thehardness increases in temperature range from 200C to 300C with theincrease in warm-forming temperatures, but decreases thereafter, showinga sharp decrease at 400C thus presenting the In such cooling, aircooling may be suitably used for a bolt of a small diameter, whereas abolt of a great diameter should be subjected to oil cooling or watercooling.

After being subjected to the method for producing a high strength boltaccording to the present invention, if a bolt is still short of thestrength required, then the bolt may be re-heated to achieve therequired strength.

For reheating, a bolt is placed in a salt bath or lead bath which hasbeen heated to a temperature of 450C to 700C, then allowed to standtherein for 5 minutes,

and quenched.

In this respect, the re-heating should be carried out at a high heatingrate and thus the bolt should be directly charged into a heating bath.The soaking temperature of the bolt should be limited to a range between450C and 700C, because a temperature over 450C can avoid the blueshortness, while a temperature below 700C can prevent a decrease instrength. While the soaking time depends on the type of heating furnace,size of bolt, heating temperatures and the like, the shorter soakingtime is preferable because of its greater effectiveness, and thuslimited up to 5 minutes.

Table shows the relationship of the mechanical properties and soakingtime when M bolt is reheated to 450C in an air atmosphere furnace, theM10 bolt being produced from a steel wire B according to the method ofthe invention.

In this connection, cooling after re-heating treatment should beaccelerated to a cooling rate higher than that of air cooling, becauseslow cooling results in an annealed bolt having a low strength, as inthe case of hotforming.

The steel wires, as used in the present invention, should include thoseas rolled or normalized, and the type of steel should cover carbon steelor low alloy steels which are generally used as materials for producingbolts. v

The rolled steels of two types, i.e., steel A and steel B, as shown inTable l, were cold-drawn'at a reductionof-area percentage of 5 to 20percent, then heated at a heating rate of 40C/sec. in an electricfurnace, warm-formed to M10 bolt and water-quenched immediatelythereafter. On the other hand, part of the bolts thus treated weredirectly charged into a salt bath which has been heated to 550C forre-heating treatment, allowed to stand therein for 20 seconds, and thenwater quenched. Those M10 bolts thus prepared were subjected toa-tensiletest using a wedge. The results thereof are shown in Tables 6 and 7.

Table 6 Results of Tensile Test Using A Wedge for Bolts of Steel A'Production Condition Wedge Angle 0 Wedge Angle l0 RemarksReductionof-area percentage Tensile Tensile in wire draw- StrengthRupture Strength Rupture ing (7r) Forming (kg/mm") Position (kg/mm).Position 5 74.1 Thread 72.6 Thread Comparative 74.5 Portion 72.0 PortionExample Warm l0 'Forming 80.0 79.2 This lnven- (550C) 80.8 78.6 tion 2086.7 83.0 This lnven- 86.l 83.9 tion 5 77.2 76.0 Comparative Warm 76.875.5 Example Forming l0 .(550C) 83.9 81.0 "lnven- This invenand Re- 83.080.6 tion Heating 20 (550C) 89.9 t 87.5 This lnven' 89.4 88.0 tion 10Cold 75.0 71.5 Prior Forming 74.5 70.5 Art Table 7 Results of TensileTest With Wedge for Bolts of Steel Production Condition Wedge Angle 0Wedge Angle l0 Remarks Reductionof-area percentage Tensile Tensile inwire draw- Strength Rupture Strength Rupture ing Forming (kglmm Position(kg/mm) Position 5 75.3 Thread 74.2 Thread Comparative 85.8 Portion 73.3Portion Example Warm l0 Forming 83.3 81.6 This lnven- (550C) 83.0 82.2tion 20 90.5 88.9 This Inven- 9l.3 89.9 tion 5 Warm 79.4 77.8Comparative Forming 78.8 76.8 Example (550C) 10 and Re- 84.6 83.0 ThisInven- Heating 85.3 82.7 tion (550C) 20 93.0 90.9 This Inven- 92.3 i f91.5 tion l0 Cold 76.5 72.5 Prior Forming 76.0 73.0 Art FIG. 3 shows thehardness distribution at the heads and shanks of M10 bolts of twogroups, one of which has been subjected in turn to cold water-drawing ata reduction-of-area percentage of 20 percent and warm- Table 8 ContinuedHeating Rate Versus Mechanical Properties (Sample steel C') forming at550C according to the present invention, 5 Heating Yield Tensile Elonga-Final Reductionand the other of which has been subjected to cold RatePoint Strength Of-area P working according to the conventional method.The (kglmmz) (kg/ninja) 4 A age test results reveal that the highstrength bolts produced, HOT/min. 851 1005. 2L] 6&8 according to thepresent invention,'as shown in Tables 59C/rpi 83.5 98-3 21.5 09.7 6 and7 and FIG. 3, present a lesser hardness difference 10 5 C/mm' 22D at thehead and shank portions of the bolts, as compared with bolts producedaccording to the conventional method as well as the bolts produced forcomparison purpose, while the former exhibits normal rupture. As isapparent from Table 8, the greater the heating The sample steels, C, Dand E, as shown in Table 1, l5 rateat warm-forming, the higherwill bethe strength. were subjected to a refining treatment of hardening andAccordingly, a heating rate of at least over 50C/min. tempering, therebyattaining the tensile strength of should be adopted for attaining thedesired increase in over 100 kg/mm to obtain high strength bolts.strength. 1

The sample steel C was oil-hardened at 870C and Sample steel C wassubjectedin turn to a refining subjected to a refining treatment oftempering at treatment of oil hardening at 870C, then tempering at 570C,thus attaining mechanical properties as a re- 570C, then: to theprelimiary cold working at a reducfined steel, as shown in Table 4.Thereafter, the steel tion-of-area percentageof 20 percent, heating to550C C was subjected to preliminary cold wire drawing at a at a heatingrate of 75C/min. in a direct electricreduction-of-area percentage of 20percent, heated at current-flowing heating means placed immediately aheating rate of 5C/min. to 40C/sec. to a tempera- 25 ahead of a pressforming machine, and hot-forming to ture of 550C, allowed to standthereat for 5 seconds, M10 bolt 5 minutes thereafter. In thisconnection, the and water quenched. The heating rates used and thecooling after warm-forming should be carried out as mechanicalproperties obtained are shown in Table 8. .rapidly as possible, sinceslow cooling results in an an- Table 2 shows the mechanical propertiesincluding nealed bolt due to self-retained heat of the bolt given thetensile strength of 10 steel wire which has been subduring-hotforming,thus failingto achieve the intended jected in turn to preliminary coldwire-drawing, rapid strengthening effect. heating and water quenching,while'the strength of the Table 9 shows the tensile test results ofbolts which bolt shown therein represents that of the shank portion havebeen subjected to three types of cooling after hotof the bolt.Accordingly, the 10 wire should present forming, i.e., water quenchingair cooling and slow properties the same as shown in Table 8, becausethe cooling, revealing that slow cooling does not give a high samplesteel C was subjected, like a 10 steel wire to the strength.

Table 9 Cooling Condition After Hot-Forming I And Mechanical PropertiesType of Cooling Yield Tensile Elonga- Final Reduc- Steel Condition PointStrength tion tion-of-area (kg/mm (kglmm 4 A percentage(%) Water 94.3110.0 19.8 67.5 S tCeel Quenching (Al Sl Air 1027 Cooling 84.0 100.021.5 68.9 Steel) Slow Cooling 71.4 85.4 23.5 77.5

preliminary cold wire drawing, rapid heating for hot- The intendedproperties may be obtained for a bolt forming, and rapid cooling. ofsmall diameter by air cooling. However, a bolt of Table 8 great diameterrequires oil-cooling or water quenching.

The sample steel C was sub ected to a refining treatment ofoil-hardening at 870C and then tempering at figiliiiigffigli 570C, thenpreliminary cold working at a reductionof-area percentage of 5 to 40percent, then heating at gzf f' g f f g f 550C at a heating rate ofC/sec., warm-forming to (kg/mum (kg/mm 4 A age 65 M10 bolt 5 secondsthereafter, and finally waterquenching. Table 10 shows the influence ofthe prelimigg ffii 23:3 182:; 13:; 23:8 nary cold working on themechanical properties ob- 180C/min. 88.3 102.0 20.0 68.5 tained.

Table Preliminary Cold Working and Mechanical Properties As can be seenfrom Table 10, the application of preliminary cold working results inthe increase in strength, whereas the toughness thereof decreases. The

preliminary cold working at a reduction-of-area percentage of over 40percent causes transverse cracking in the refined steel material, thusinterrupting further working. a

The sample steel C was in turn subjected to a refining treatment ofoil-hardening at 870C and then tempering at 570C, then preliminary coldworking at a reduction-of-area percentage of 20 percent, heating to 550Cat a heating rate of 75C/sec. in a direct current-flowing heating meanspositioned immediately ahead of a press forming machine, warm-forming toa M10 bolt 5 seconds thereafter, and then water-quenching. FIG. 4

shows hardness distributions at the head and shank portions of bolts.

In general, the head of a bolt has been subjected to 75 percent working,whereas the shank portion has been subjected to only 20 percentpreliminary working,

such that the difference in such working percentage is reflected in tactto the hardness difference. As a result, the bolt head exhibitsabnormally high hardness (strength) as compared with those of the shankportion, while the toughness thereof decreases accordthe presentinvention presents lowered hardness for the head portion, because, whenwarm-formed as shown in FIG. 2, the deformation resistance of the samplesteel will decrease with the accompanied lesser workhardening, while theshank portion of the bolt presents a hardness increase due to thestrengthening treatment of the invention, i.e., a cycle of thepreliminary cold working at a reduction-of-area percentage of 20percent, rapid heating used for warm-forming and rapid coolingthereafter, thereby lessening the difference in hardness between thehead and shank portions of the bolt.

The sample steels D (boron steel) and E (Cr-Mo steel) were subjectedtwice to a cycle of hardening treatment by a high-frequency heatingmeans, then a tempering treatment in a head bath, preliminary coldworking at a reduction-of-area percentage of 20 percent, heating to 550Cat a heating rate of C/sec. and warm forming to M10 bolt.

M10 bolt was formed with a notch of 0.05R (stress concentration factor6) and was subjected to a socalled loop type delayed rupture test in ahighhumidity atmosphere, in which the bolt was immersed in a water bath,with the tension being applied thereto. Table 11 shows the results ofsuch a test.

As is clear from Table 11, in case a bolt is produced by hot-formingaccording to the strengthening treatment of the invention, after theaustenite crystal grain size has been rendered finer by means of therefining treatment in combination with the high frequency inductionheating treatment, the bolt thus produced presents excellent resistanceto delayed rupture, as compared with a conventional bolt having the samestrength.

As is apparent from the foregoing description, a high strength boltaccording to the present invention presents a high tensile strength ofover kg/mm without impairing the toughness thereof, in contrast to theconventional heat-treated bolt.

On the other hand, since rapid heating in the high frequency inductionheating means used for the refining treatment for preliminary coldworking can render finer the austenite crystal grain size, the boltsproduced according to the present invention can present excellentresistance'to delayed rupture, as compared with conventional highstrength bolts.

Table 1 1 Results of the Delayed-Rupture Test Mechanical PropertiesSample Steel Heat-treatment Yield Tensile Elonga- Final Reduc- AusteniteNotch Rupture Point Strength tion tion-of-area Grain Size Strength Time(kglmm (kglmm *1 percentage *2 (kg/mm *3 (HV) D-O (compara- .Formed to abolt tive material) and then heat- 130.8 141.3 17.6 55.2 7.0 208.5 50

treatment, 850C 0.0. 420C A.C. D-l (this High frequency ininvention)duction heating heat 135.6 137.6 18.3 59.1 11.8 220.3 3256 treatment 20%preliminary working W.l-l. 850C 00. 380C A.C. 500C W.l-l.

Table .11 -Continued Results of the Delayed-Rupture Test MechanicalProperties Sample Steel Heat-treatment Yield Tensile Elonga- FinalReduc- Austenite Notch Rupture Pomt Strength tion (72) tion-of-areaGrain Size Strength Time (kg/mm) (kg/mm) *1 percentage (71) *2 (kglmm *3(HV) E-O (compara- Formed to bolt and tivc material) thenheat-treatment, 128.2 138.3 15.6 53.4 8.0 207.0 43

850C 0.0. 490C A.C. E-l (this High frequency ininvention) ductionheating heat 137.8 138.1 18.3 58.0 12.0 225.0 8634 treatment 209?preliminary working W.H. 850C 0.0. 490C A.C. 500C W.H.

*1 Elongation was measured by gauge length 4 A; "2 Grain size afterheat-treatment:

"3 The rupture time in the delayed rupture test is carried out under thenominal load of 175 kg/mm and using the water immersion at roomtemperature.

treatment of oil-hardening at 870C and tempering at 650C coldwire-drawing at a reduction-of-area percentage of percent, heating to550C at a heating rate of 240C/sec., holding at said temperature for 5seconds and water quenching.

The material thus treated was then subjected twice to a strengtheningtreatment consisting of cold working at a r'eduction-of-area percentageof 20 percent, heating to 500C at a heating rate of 240C/sec., holdingat said temperature for 5 seconds, and water quenching.

The materials, which have been subjected to one and two cycles ofstrengthening treatment of the invention as described, were furthersubjected in turn to a wire drawing at a reduction-of-area percentage of20 percent, heating to 500C at a heating rate of 75C/sec., warm-formingto M10 bolt 5 seconds thereafter and water quenching. Y

The mechanical properties obtained are shown in Table 12.

Table 12 Mechanical Properties Material Worked Yield Tensile Elonga-Final Re- Point Strength tion duction-of- (kg/mm) (kg/mm) 4 A areapercentage Refined Material One cycle of strengthening treatment Twocycles of strengthening treatments M10 bolt of refined material M10 boltsubjected to one cycle of strengthening treatment M10 bolt subjected totwo cycles of strengthening treatments As can be seen from Table 12, incase the refined tensile strength and yield point. In addition, the testresults show that the repeated strengthening treatments can presentfurther increased tensile strength for bolts which have beenwarm-formed.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is to beunderstood therefore that within the scope of the appended claims, thepresent invention -may be practiced otherwise than as specifically de- 4the steps of:

subjecting said material to cold working at a reduction-of-areapercentage of 10 percent and over;

rapidly heating said material thus worked to a temperature range from450C to the A transformation point and at a rate of at least 50C/min.;

warm-forming said material to a bolt shape; and

air cooling said material or cooling said material at a cooling ratehigher than that of said air cooling, said cooling rate being at least50C/min.

2. A method as defined in claim 1, wherein a carbon steel or a low alloysteel material is subjected at least once to the strengthening treatmentconsisting of cold working at a reduction-of-area percentage of 10percent and over, rapid heating to a temperature range from 250C to theA transformation point, and rapid cooling, after which said materialthus treated is subjected in turn to cold working at a reduction-of-areapercentage of 10 percent and over, rapid heating to a temperature rangefrom 450C to the A transformation point; warm-forming to a bolt shape,and air cooling or cooling at a cooling rate higher than that of saidair cooling.

3. A method as defined in claim 1, wherein the high strength boltproduced according to claim 1 is subjected in turn to reheating to atemperature range from 450C to the A transformation point, air-coolingor cooling at a cooling rate higher than that of said air cooling.

4. A method as defined in claim 3, wherein the heating time for bolts atthe reheating temperature during the reheating treatment is within 5minutes.

5. A method as defined in claim 1, wherein said steel material is thatwhich is hot-rolled or has been subjected to a normalizing treatment orannealing treatment after hot-rolling, thereby presenting a pearlitestructure.

6. A method as defined in claim 1 wherein said steel material is thatwhich has been subjected to hardening and tempering treatments afterhot-rolling, thereby presenting a tempered martensite structure andwherein the reduction-of-area percentage for cold working used rangesfrom to 40 percent.

7. A method as defined in claim l, wherein said steel material issubjected at least once to a cycle of hotrolling, rapid heating andhardening, thereby rendering finer the austenite crystal grain size toover No. 10 and inclusive according to ASTM grain size, after which saidmaterial is subjected to a tempering treatment, thereby presenting atempered martensite structure, and wherein the reduction-of-areapercentage of cold working ranges from 10 to 40 percent.

8. A method as defined in claim 1, wherein the content of carbon is 0.5percent, or lower.

9. A method as defined in claim 1 wherein the cold working iswire-drawing.

10. A method as defined in claim 1, wherein the heating time for boltsat the heating temperature during the strengthening treatment is within3 minutes.

1. A METHOD FOR PRODUCING A HIGH STRENGTH BOLT FROM A CARBON STEEL OR A LOW ALLOY STEEL MATERIAL, COMPRISING THE STEPS OF: SUBJECTING SAID MATERIAL TO COLD WORKING AT A REDUCTION-OFAREA PERCENTAGE OF 10 PERCENT AND OVER; RAPIDLY HEATING SAID MATERIAL THUS WORKED TO A TEMPERATURE RANGE FROM 450*C TO THE A1 TRANSFORMATION POINT AND AT A RATE OF AT LEAST 50*C/MIN.; WARM-FORMING SAID MATERIAL TO A BOLT SHAPE; AND AIR COOLING SAID MATERIAL OR COOLING SAID MATERIAL AT A COOLING RATE HIGHER THAN THAT OF SAID AIR COOLING, SAID COOLING RARE BEING AT LEAST 50*C/MIN.
 2. A method as defined in claim 1, wherein a carbon steel or a low alloy steel material is subjected at least once to the strengthening treatment consisting of cold working at a reduction-of-area percentage of 10 percent and over, rapid heating to a temperature range from 250*C to the A1 transformation point, and rapid cooling, after which said material thus treated is subjected in turn to cold working at a reduction-of-area percentage of 10 percent and over, rapid heating to a temperature range from 450*C to the A1 transformation point; warm-forming to a bolt shape, and air cooling or cooling at a cooling rate higher than that of said air cooling.
 3. A method as defined in claim 1, wherein the high strength bolt produced according to claim 1 is subjected in turn to reheating to a temperature range from 450*C to the A1 transformation point, air-cooling or cooling at a cooling rate higher than that of said air cooling.
 4. A method as defined in claim 3, wherein the heating time for bolts at the reheating temperature during the reheating treatment is within 5 minutes.
 5. A method as defined in claim 1, wherein said steel material is that which is hot-rolled or has been subjected to a normalizing treatment or annealing treatment after hot-rolling, thereby presenting a pearlite structure.
 6. A method as defined in claim 1 wherein said steel material is that which has been subjected to hardening and tempering treatments after hot-rolling, thereby presenting a tempered martensite structure and wherein the reduction-of-area percentage for cold working used ranges from 10 to 40 percent.
 7. A method as defined in claim 1, wherein said steel material is subjected at least once to a cycle of hot-rolling, rapid heating and hardening, thereby rendering finer the austenite crystal grain size to over No. 10 and inclusive according to ASTM grain size, after which said material is subjected to a tempering treatment, thereby presenting a tempered martensite structure, and wherein the reduction-of-area percentage of cold working ranges from 10 to 40 percent.
 8. A method as defined in claim 1, wherein the content of carbon is 0.5 percent, or lower.
 9. A method as defined in claim 1 wherein the cold working is wire-drawing.
 10. A method as defined in claim 1, wherein the heating time for bolts at the heating temperature during the strengthening treatment is within 3 minutes. 